(1) Field of the Invention
The present invention relates to a video server which can transmit a video as per request among a plurality of videos stored therein.
(2) Description of the Related Art
There has been active research for an interactive television system in the field of CATV (Cable Television) system, and the core of the system is a video server.
In the interactive television system, each subscriber requests a video (program) he would like to watch to the CATV station, and the station transmits the videos as per requests to individual subscribers. Thus, the interactive television system enables the subscribers to enjoy the videos at any convenient time once it is put into practical use.
Among various applications of the interactive television system, the on-demand television system has been already made available to the public. The on-demand television system comprises a video tape library, a plurality of video tape players for playing the video tapes, a plurality of TV (television) cables for transmitting a plurality of videos being played to terminals equipped at the subscribers' end, and a switch for switching the TV cables.
The entire system is controlled manually by a plurality of operators at the station. To be more specific, upon receipt of the requests from the subscribers, the operators connect the TV cables to the their terminals by means of the switch, and play the video as per their requests by means of the video tape players. Accordingly, each video being played is transmitted to their respective subscribers via the connected TV cables.
However, such a manual control limits the service of the on-demand television system to a small number of subscribers. For example, it is well assumed to receive 100 requests at a time if there are 1000 subscribers. However, it is almost impossible to manage all the requests simultaneously under the manual control; there is a considerable lag time between the time the requests are received and the transmission of the requested videos.
Given these circumstances, the pure-on-demand television system was proposed in "Nikkei Communication" No. 144, pp. 38-55, Feb. 15, 1993, and "Nikkei Electronics", No. 584, pp. 58 as an ideal interactive television system, and it has been attracting public interests.
The pure-on-demand television system comprises a storage unit for storing digital data of the videos, a plurality of cables for transmitting the digital video data to individual subscribers, a plurality of decoders for generating visible images by decoding the transmitted digital video data, and an ATM (Asynchrnous Transfer Mode) switch for switching the cables.
Since the videos are transmitted in the form of digital data, they are processed quickly, and the subscribers can not only play the videos in real time whenever they want, but also play the videos quickly in both directions (fast-forward and fast-rewind) in real time as if they were manipulating the video tape player.
However, even with this ideal system the problem of the time lag is not solved. This is because a myriad of requests are sent to one storage unit at a time, and it takes time to read out the requested video data from the storage unit. In addition, it takes time to transmit the readout data to the subscribers by the means of the ATM switch when there is considerable traffic therein. Thus, the time lags caused in both reading out and transmitting the video data make the real time transmission impossible.